This invention relates generally to a laminate film suitable for packaging, especially food packaging, and relates more particularly to a heat shrinkable laminate film having especially high oxygen barrier properties.
In the packaging of food in thermoplastic film, it is frequently desirable that the film provide an oxygen barrier, i.e. have a low permeability to oxygen. For example, film made of a copolymer of 65 to 95% by weight of vinylidene chloride and 5 to 35% of a vinyl comonomer such as vinyl chloride, acrylonitrile or methyl acrylate, generally referred to as saran, is known to give a good oxygen barrier. The term "high barrier vinylidene chloride copolymer" as used herein is intended to refer to a saran having at least about 85% by weight of vinylidene chloride. The oxygen barrier property of vinylidene chloride copolymers tends to increase with increasing content of vinylidene chloride.
Additionally, it is frequently desired in the packaging of food that the packaging film be heat shrinkable so that the film may be shrunk snugly about a contained product thereby presenting an attractive package appearance. In the manufacture of heat shrinkable film, it is commercially advantageous to produce packaging film in tubular form, e.g. by the conventional blown bubble method, and then to stretch orient such film to render it heat shrinkable by the conventional trapped bubble method. In many cases, such film is irradiatively crosslinked before orientation to enhance orientability. Representatively, such processes are disclosed in U.S. Pat. No. 3,741,253 issued June 26, 1973 to Brax et al.
The term "high barrier" film as conventionally used refers to an oxygen barrier thermoplastic material composed predominately of the barrier constituent to the extent that the material is brittle and relatively difficult to stretch orient. Generally, the higher the barrier constituent, the higher the barrier property, but the higher the degree of crystallinity which tends to embrittle the film to impart corresponding low impact strength.
One approach to making a high barrier, heat shrinkable multilayer film has been to melt-form the composite film and then to stretch orient the film before crystallization of the high barrier material proceeds to an adverse extent. An alternative approach provides for lamination of stretch-oriented barrier film to a heat shrinkable, base film, i.e. the high barrier film is stretch oriented after melt forming and before crystallization proceeds to an adverse extent. Apparently, stretch orientation of nascent high barrier material reduces the size of crystalline regions of the microstructure so that the material does not become brittle. In this alternative approach, it would be desirable to eliminate the stop of stretch orienting the nascent high barrier film. This is the problem with which the present invention is concerned.
Of general interest are the disclosures of U.K. Pat. No. 1,591,423 for "Laminate Film of Polyethylene and Vinylidene Chloride Copolymer" and U.K. Pat. No. 1,591,424 for "Process of Heating Food in a Package of Polymeric Laminate Films" by Thompson published June 24, 1981, which disclose a laminate film formed by coating vinylidene chloride copolymer, other than by melt extrusion, onto the surfaces of a heat shrinkable film.